Documents relating to methods and devices for locating a magnetic object, such as document FR 2 988 862, are known.
Document FR 2 988 862 discloses a method for locating a magnetic object from measurements taken by an array of tri-axial magnetometers, this array including N tri-axial magnetometers that are mechanically connected to one another with no degree of freedom in order to preserve a known distance between each of these magnetometers, N being an integer higher than five. The array of magnetometers is comprised in a device comprising a bearing surface for a writing medium such as one or more sheets, which are optionally joined.
The location of a magnetic object may be considered to be its position (its (x, y, z) coordinates in a coordinate system that is fixed with respect to the array of magnetometers) and the value of its magnetic moment ((Mx, My, Mz) in the same coordinate system), which is representative of the orientation of the magnetic object.
The introduction of a bias into the measurements used in this type of method makes the location imprecise, or even unstable.
In the context of a mobile use of such a locating device, i.e. when the user is able to move the locating device bearing its writing medium, its orientation may be modified, this leading directly to a rotation of the Earth's magnetic field in the coordinate system measured by the array of magnetometers, and therefore to a bias in the measurements of all the magnetometers.
Movement of the locating device in a uniform magnetic field therefore makes it almost impossible to use such a locating device.
When such a device is used in a system for digitizing handwritten text, in which type of system the precision with which it is sought to determine the location is better than 1 mm, only a slight variation in the Earth's magnetic field in the coordinate system of the magnetometers leads directly to a measurement deviation larger than the sought precision.
It is known from the prior art to eliminate, on turn-on, the measurement bias in all the sensors, or in the absence of any magnetic object.
Such a method does not allow movement during the location of a magnetic object and therefore does not solve the underlying problem mentioned with respect to a mobile use. In particular, in the context of a writing digitization application, it is frequently the case that the user will move the locating device in order to obtain a feasible writing angle while modifying his posture and moving.
It is also known to place a magnetometer at a sufficient distance from the movable magnetic object, so that this magnetometer is not affected by the magnetic field generated by the movable magnetic object. The sealed measured by this distant magnetometer serves to suppress the magnetic component induced by the Earth's field.
Such a method does not allow devices of small size to be used, and therefore does not lend itself well to a portable device. Furthermore, this method is very sensitive to errors in the measurements of this distant magnetometer and in particular to malfunction or to disruption of the latter by magnetic fields other than the Earth's.